X- and gamma-ray sensitive image intensification tube

ABSTRACT

The invention relates to an X- and Gamma-ray sensitive image intensification tube for use in a so-called gamma camera or the like, which tube preferably has a single crystal input phosphor.

CROSS REFERENCES TO RELATED APPLICATIONS

1. Application Ser. No. 578,617, filed Sept. 12, 1966, by Carl W. Hansenunder the title "Method and Apparatus for Producing an Amplified Imageof the Distribution of Gamma Radiation or the Like."

2. Application Ser. No. 594,083, filed Nov. 14, 1966, now U.S. Pat. No.3,543,384, by Carl W. Hansen under the title "Collimators and Methods ofCollimator Fabrication."

BACKGROUND OF THE INVENTION

1. Field of the Invention

Radioactive isotopes are used in a variety of medical diagnostictechniques. With some of these techniques, a radioactive isotope isadministered to the patient. Later, a study is made of the distributionand concentration of the isotope in the patient. This type of study isof benefit in diagnosing tumors and other ailments.

Prior to this invention and the gamma camera of the first referencedcopending application, scintillation scanners were used in the usualtechniques employed for the visualization of the spatial distribution ofthe radioactive material selectively absorbed in the tissues of thesubject. In the usual scintillation scanning technique, a scintillationprobe is moved along a series of parallel paths over the portion of thesubject's anatomy that is being studied. Radiation from an administeredradioactive isotope causes the probe to impart electrical impulses to arecording apparatus. This recording apparatus produces a graphic imageof the spatial distribution of detected radiation. Typically, images areproduced on paper by various types of recorders and on film bytechniques known in the art such as those disclosed in U.S. Letters Pat.No. 3,159,744, entitled "Scintillation Scanner Photorecording Circuit"and U.S. Letters Pat. No. RE 26,014 entitled "Scintillation Scanner."

Because the probe is moved along these parallel paths, the time forconducting a complete scan is relatively long which results in a numberof disadvantages. These include:

1. Relatively few patients may be examined with one apparatus;

2. The protracted period of time required results in discomfort of thepatient; and

3. The image does not show the total distribution of radioactivesubstance in the subject matter at any one time.

Another type of proposed image system is sometimes referred to as astationary scanner. This scanner uses a matrix of scintillatorspositioned over the area to be studied. Each scintillator within thematrix is associated with a specific location in the area to be studied.Light impulses generated by different scintillators in the matrix areread out by photomultiplier tubes. Impulses emitted by these tubes maythen be used to reproduce the image formed by the radiation in a mannersimilar to that used in the scanning system described in the referencedpatents. Although the matrix system is faster than scanning methodsprior to it, the system is limited by the number of scintillators andphotomultiplier tubes which can practically be used. Accordingly,resolution is poor.

A variation of the scanner using a matrix of scintillators is one with ascintillator positioned behind an apertured collimator. A plurality ofphotomultiplier tubes are positioned on the side of the scintillatoropposite the collimator. Each time the scintillation appears on thescintillator it is viewed by a plurality of the photomultiplier tubes.The resulting impulses from the photomultiplier tubes are passed througha computer which determines the spatial location of the scintillation onthe scintillator according to the relative strengths of the plurality ofelectrical pulses resulting from the scintillation. A graphicreproduction of the scintillations is then produced on the oscilloscope.Such devices are obviously complex and susceptible to some error due tovariations such as from one multiplier tube to another. The spatiallocation of each scintillation is uncertain because of the poorstatistical accuracy of each electrical signal. For these and otherreasons such devices tend to fail to accurately demonstrate the truespatial distribution of radioactivity in the subject under study.

In the first referenced, copending application, another type ofstationary scanner known as a gamma camera is described. There an imageintensification tube is stimulated by photons emitted by the radioactivematerial administered to the patient. The image tube has an inputphosphor which is stimulated by these photons and which emits light inresponse to them. The light emitted by the input phosphor stimulates anelectroluminescent layer which emits electrons. These electrons areaccelerated electronically against an output phosphor. The intensifiedimage produced by the output phosphor then passes through a lightamplification stage into a closed-circuit television system.

2. Description of the Prior Art

Image tubes in the prior art generally comprise an evacuated envelopewith an input "sandwich" at one end of the tube, an output phosphor atthe other end, and a means to electronically accelerate electronsemitted by the sandwich against the output phosphor.

This input sandwich typically includes a dishlike aluminum member whichserves multiple purpose of: (1) filtering the input energy; (2)preventing external light from stimulating the input phosphor; (3)mechanically supporting the balance of the sandwich; and (4) a physicalbarrier against migration of molecules from the front of the inputphosphor layer into the vacuum within the envelope.

The typical input phosphor is made up of many discrete particles of aphosphorescent material admixed in a suitable bonding and sealingvehicle such as an epoxy resin material. This admixture forms a slurrywhich is deposited in a thin layer on the inner surface of the aluminumdish to provide a phosphorescent layer of substantially uniformthickness with the phosphorescent material sealed off by the bondingagent or other barrier layer to prevent its migration into anelectron-emissive layer or into the vacuum in the tube.

An electron-emissive layer is then deposited on this input phosphor andbarrier layer. Typically, a microscopically thin film of metal will beinterposed between the barrier layer and the electron-emissive layer toprovide replenishment of electrons for the emissive layer. The metallayer is microscopically thin so that it does not materially inhibit thepassage of light from the phosphor layer to the electron-emissive layer.

Electrons emitted by the emissive layer are accelerated by a suitablemeans such as electrostatic rings or a metal "jacket" which surroundsthe acceleration path and which carries an electrical charge.

While image tubes of this type have been satisfactory for so-calledbright fluoroscopic studies where an X-ray source provides thestimulating energy they severely limit the use of a gamma camera,especially when that gamma camera is utilized for conducting medicalstudies on human beings.

One reason image tubes of this type have limited the use of gammacameras is that they have a very low so-called conversion efficiency.That is, a relatively low percentage of photons of energy entering theimage tube are actually converted to light signals. One reason for thisis that the typical input phosphor is relatively thin and much of thegamma energy passes through the phosphor without causing the emission oflight.

The expedient of making the input phosphor thicker by prior knowntechniques has not been a satisfactory solution for gamma cameras. Onereason it has not been a satisfactory solution is that eachphosphorescent particle used to build up a fluorescent screen by thisdescribed technique has reflective surfaces. When enough particles areused to make a screen of a thickness which is efficient in convertingthe input energy to light, there are many reflective surfaces. Becausethere are so many surfaces, the stimulated light may reach theelectron-emissive layer at a location which is, in a plane parallel tothe plane of the input phosphor, a substantial distance from its source.

Obviously, this diversion of light will result in very poor resolutionwith the result that a produced graphic image will not provide anaccurate indication of the true spatial distribution of the isotope inthe subject under investigation.

While it has been known that halogens are efficient phosphors, they havenot been accepted for use in image intensification tubes and the likebecause the halogens tend to disassociate relatively easily. This isespecially true with iodides. A disassociated halogen atom tends to"poison" the vacuum by chemically interacting with the photo cathode;i.e., the electron-emissive layer, and even the output phosphor to causedeleterious effects and early tube failure.

Another problem has been how one can provide an input phosphor of thesize requisite for a gamma camera or the like which produces accurateand dependable results with the relatively low stimulating energyavailable from a radioactive isotope. The above-described input phosphorlayer is in reality a mosaic of discrete phosphorescent particles.Proposals have been made to develop a mosaic of larger pieces byadhering together a series of relative large crystals. According tothese proposals, the crystals in the mosaic are positioned and adheredin a dishlike configuration which is generally parabolic in shape inorder for the input "sandwich" to be focused relative to the electronaccelerating field. These proposals also call for encapsulation of themosaic to prevent poisoning of the image tube.

One major shortcoming of any mosaic is the conversion efficiency of eachelement of the mosaic is different from the others. The result is that agiven light output of one crystal of the mosaic is not reflective of thesame amount of incident energy as is the same light output from anothercrystal of the mosaic. This is true even where all crystals of themosaic are cut from the same original crystal. The reason this is trueis that the surfaces on each crystal cleaved from the large crystal isdifferent from the surfaces on the other crystals, with differentreflective indices. This results in a different light output in onecrystal as compared to another for any given amount of incident energy.

SUMMARY OF THE INVENTION

It has been discovered that all of the above-described difficulties canbe overcome and that it is possible to produce an image tube with goodresolution in which there is conversion of a high percentage of inputgamma photons to light. This is accomplished through the provision of aninput phosphor which;

1. Is transparent to its own light.

2. Is a good absorber of the ray energy to which it is exposed;

3. Is photoluminescent;

4. Mates well chemically with the electron layer such as the well-knownlayer sold commercially under the designations S-11 (monoalkalis) andS-20 (trialkalis). Present preference, based on test data available andbecause of its stability and efficiency is S-20 which is a trialkali ofantimony and three alkaline metals, potassium, sodium and cesium, whichhave been simultaneously vaccum-deposited in place;

5. Emits light energy which stimulates the electron layer efficientlyand thus the phosphor must mate well in terms of the level of its lightenergy with the electron-emissive layer;

6. Is capable of being made into a dish shape so that the emittedelectrons are focused with respect to the accelerating field;

7. Withstands the vacuum well without poisoning the interior of theimage tube;

8. Withstands bake-out temperatures used to clean the tube before it issealed without distortion or other degradation of the crystal. Typicalbake-out temperatures may be as high as 300° F.; and

9. Is a unitary sheet with uniform energy converting properties over itsentire area and preferably a single crystal. This permits the phosphorto be considerably thicker (i.e. .060 inch) than the prior art whilereasonably good resolution is maintained. The increased thickness andabsorption coact to produce a phosphor that is preferably capable ofabsorbing about 30 percent of gamma energy emitted by a 150 kev.(kiloelectron volts) source and proportional percentages of otherso-called low energy isotopes, as compared with the 3 percent at 150kev. on a typical prior phosphor. As used here, the term "low energy"means 250 kev., or less.

It has been discovered that cesium iodide is ideal for this purpose,fits all of these criteria, and does not, as has been feared withrespect to all halogens, poison the tube. In fact, the requirements forphysical barriers to prevent migration of molecules within the tube areactually reduced as compared with conventional prior art phosphors suchas zinc cadmium sulfide which has most commonly been employed in theprior art. This further improves the performance of the tube, especiallyas compared with prior art devices that employed glass as a barrierlayer, because energy losses due to absorption of violet and ultravioletlight by the barrier are substantially if not completely eliminated.

Rolled sheets of cesium iodide are available which, although in realitya type of mosaic, are unitary sheets which produce improved results. Thebest results, however, are obtained with a single crystal of cesiumiodide which is transparent as compared to the somewhat yellow color ofthe rolled sheet and which has uniform light converting efficiencythroughout. Thus, a single crystal of cesium iodide fits all of theabove criteria.

A limited number of additional phosphors substantially fit thesecriteria generally but provide other disadvantages. For example, sodiumiodide is available in large single crystals but to provide the dishlikeshape it must be machined as compared with cesium iodide which may beheat softened and molded. Further, sodium iodide is highly hygroscopicand therefore must be dried and sealed carefully. Further, it is soactive chemically that it must be isolated carefully within asurrounding vacuum envelope. Potassium iodide, while not of thehygroscopic nature of sodium iodide, has, in other respects, the samedrawbacks as sodium iodide. These phosphors which achieve the criterionset out above and are utilized with the present invention may becollectively referred to as single crystalline sheets. Thus, as usedherein the term "single crystalline" encompasses the described unitarysheets such as the described roll sheet of cesium iodide and thedescribed single crystals as distinguished from prior image tube inputscreens such as those made up of small particles adhered in place withan epoxy adhesive.

Accordingly, the object of this invention is to provide a novel andimproved gamma camera and more particularly a novel and improved imageintensification tube which is especially suited for use in a gammacamera.

Other objects and a fuller understanding of the invention may be had byreferring to the following description and claims taken in conjunctionwith the accompanying drawing.

DESCRIPTION OF THE DRAWING

The drawing is a schematic representation of a gamma camera includingthe improved image tube with the image tube being shown in crosssection.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawing, a subject under investigation is showngenerally at 10. The subject, in the case of a medical study, might forexample be a thyroid gland. With a gamma camera, the administeredisotope is preferably a so-called low energy isotope, such as iodine125.

Gamma radiation emitted by the source 10 passes through a parallel holecollimator indicated diagrammatically at 11. Preferably, the collimatoris of the type described in the second above-referenced copendingapplication. After the gamma radiation passes through the collimator 11,it enters an image tube shown generally at 12. The image tube will bedescribed in greater detail below.

The output of the image tube 12 is focused by a suitable optical lens 13onto input end 14 of a light amplifier 15. The diagrammaticallydisclosed light amplifier 15 is a two-stage amplifier. Because of thevery high efficiency of the image tube of this invention, in manystudies the light amplifier may be single stage or eliminated entirely.The output of the amplifier passes through a further optical lens 17which focuses the light amplifier output onto input end 18 of atelevision pickup tube 19. The disclosed pickup tube 19 is an orthiconconnected to an orthicon chain 20. The orthicon chain includes the usualvideo amplifier and sync and blanking generators.

The output of the orthicon chain 20 may be transmitted to a storagedevice 22 and then to a television monitor 23. The components of thisstructure which converts the light output of the image tube to an imagetube on the television monitor and other suitable allied structure isdisclosed in a more detailed manner in the first referenced copendingapplication.

The image tube 12 includes a typical glass envelope 25 composed of twosections clamped together at 26, 27. The glass envelope is equipped withvacuum nipples, not shown which, in manufacture, are used to evacuatethe tube and then sealed. The glass envelope 25 has an enlarged inputend window 29. An input phosphor layer 30 is provided near the inputwindow 29.

As noted above, this phosphor layer 30 is a single crystalline sheet,preferably a single crystal of cesium iodide molded to the desiredsubstantially parabolic configuration. Because of its relative thicknessand because the input phosphor is a unitary sheet rather than a seriesof particles bonded together by a plastic material or the like, andbecause of its relatively stable characteristics in vacuum, the inputphopshor of this invention does not require the dishlike aluminumelement used in the prior art.

Thus, of the four purposes of the aluminum dish described above, all butbarring the entrance of light have been omitted. Obviously, the window29 of the envelope 25 may be darkened if desired or the device may beused in a darkened room to prevent adverse effect on the image tube byambient light.

Since the reasons for the aluminum dish are eliminated, it can beeliminated. With the present device, the input phosphor is supported bya mounting and support member in the form of a ring 32 at the perimeterof the input phosphor. The ring 32 may be supported by suitable studs33, 34 suitably secured to the input window.

An electron-emissive layer 36 is provided. The electron-emissive layer36 may be, as noted above, a trialkali. As disclosed, the unitaryphosphor layer 30 and the electron-emissive layer 36 are shown separatedby a suitable transparent barrier layer 37. As noted above, one may beable to omit the barrier layer with this particular combination of asingle crystal cesium iodide input phosphor and a compatibleelectron-emissive layer, however, longest tube life appears to beassured by providing a very thin, clear, light transparent barrier layerof the plastic. Obviously, the manufacturing requirements for thebarrier layer are considerably reduced as compared with the prior artbecause a flaw in the barrier layer will not result in the usuallyrelatively abrupt tube failure which may be experienced with prior artbarrier layers.

Electrons emitted by the emissive layer 36 are accelerated by anelectrostatic field established by a conductive wall coating 38 on theinterior of the envelope 25. This coating 38 accelerates and focuses theelectrons through an anode thimble 39 onto an output phosphor 40. Lightemitted by the output phosphor 40 is focused by the lens 13 onto thelight amplifier 15 and results in an image in the monitor 23 in themanner described above.

Although the invention has been described in its preferred form with acertain degree of particularity, it is understood that the presentdisclosure of the preferred form has been made only by way of exampleand that numerous changes in the details of construction and thecombination and arrangement of parts may be resorted to withoutdeparting from the spirit and the scope of the invention as hereinafterclaimed.

I claim:
 1. In an image-forming device including an imageintensification tube, .Iadd.said tube including an evacuated envelope,.Iaddend.and .Iadd.said device further including .Iaddend.means toreproduce an image output from said tube, an improved image tube inputphosphor layer comprising:a. a single crystalline sheet .Iadd.of cesiumiodide, said sheet being .Iaddend.of substantially uniform thickness.Iadd.and being located within said evacuated envelope; .Iaddend. b.said sheet having the following physical properties:i. photoluminescentand transparent to its own light; ii. efficient in absorption of rayenergy; iii. of sufficient thickness and absorption to convert to lightenergy substantial percentages of gamma energy emanating from a sourceof energy of 250 kev. or less; iv. of high resolution characteristics;and v. dish-shaped in configuration and focused to an acceleratingelectron field in said tube. .[.2. The image device of claim 1 whereinsaid sheet is cesium iodide..].3. The image device of claim 1 whereinsaid sheet is a single crystal. .[.4. The image device of claim 1wherein said sheet is a single crystal of cesium iodide..].
 5. An imageintensification tube responsive to stimulation by gamma energy or thelike comprising:a. an evacuated envelope having an input end and anoutput phosphor spaced therefrom; b. a dish-shaped light-emissive layernear said input end and composed of a single crystalline sheet ofphotoluminescent material; c. an electron-emissive layer deposited nearan inner surface of said light-emissive layer such that light photonsemitted by said light-emissive layer stimulate said electron-emissivelayer and cause the latter layer to emit electrons; d. means foraccelerating electrons from said electron-emissive layer against saidoutput phosphor; e. said dish shape of said light-emissive layer andsaid electron layer being substantially contoured to the shape of anaccelerating electron field produced by said acceleration means so as tobe focused therewith, and f. a mounting means supporting saidlight-emissive layer and electron-emissive layer near said input end. 6.The tube of claim 5 wherein said photoluminescent material is a singlecrystal of cesium iodide.
 7. The tube of claim 5 wherein saidphotoluminescent material is a single crystal.
 8. The tube of claim 5wherein said photoluminescent material is cesium iodide.
 9. The tube ofclaim 5 wherein said photoluminescent material is of sufficientthickness to absorb about 30 percent of source energy of 150 kev. orless and is transparent to its own light.
 10. The tube of claim 5wherein said photoluminescent material can withstand temperatures of300° F. without distortion or other degradation.
 11. The tube of claim 5wherein said mounting means is open at a central portion wherebyentering energy passing through said central portion is not filtered orotherwise obstructed by said mounting means. The method of forming aninput gamma ray sensitive structure for an image intensification tubecomprising the steps of:a. heat softening a single crystal of cesiumiodide; b. molding the single crystal of cesium iodide while heatsoftened to a dish-shaped configuration having a concave output surfaceto form a photoemissive layer; c. depositing an electron-emissive layeron the output dish of said formed dish-shaped cesium iodide crystal withthe electron layer and crystal in sufficiently close juxtaposition thatlight energy emitted by the crystal stimulates said electron-emissivelayer and causes the emission of electrons when in use; and d. securinga mounting means to the periphery of said photoemissive layer andmounting said layers in an envelope with the convex surface of thephotoemissive layer oriented toward an input end of said envelope. 13.The method of claim 12 wherein the depositing steps is on a barrierlayer and including the step of providing a barrier layer between theelectron-emissive layer and the crystal. .Iadd.
 14. The image-formingdevice of claim 1 wherein said sheet is of sufficient thickness toabsorb approximately thirty percent of source energy at 150 kev. orless. .Iaddend.